Hi Robert. For the first part of your question, are you referring to the light output? If so, that would depend on the bulb, wattage, headlight style, etc. As Calixtus explained very well above, high-quality light produced by LEDs generally do not degrade over time, unlike halogens.

These are complete $#*!. You cannot see on the highway at night!

One of my bulbs went to 0 output after 2 weeks. If i pay them $20 I can get a bulb replaced "under warranty"

Well I'm no scientist but as a car enthusiasts who has owned close to 100 different vehicles in my lifetime these are some of the best headlight and fog light bulbs I've ever used and won't use any other type of bulb from any of their competitors. Come on who in this day and age still overs a lifetime warranty

Cold start is the best and brightest. As current increases in the LED, so does the velocity of the electrons passing through the semiconductor, thus increasing intensity... As voltage decreases, the forward current decrease; in this way the intensity of emitting light also decreases. LEDs are long-life light sources which generally do not suddenly fail, but gradually lose their brightness and performance over time... In some poorly designed LED lights, bad heat sinking can result in overheating, which in turn could cause the LED to completely burn out or increase the rate of decline in brightness